c++ - avoiding deadlock when pthread_cond_wait and pthread_cond_signal -

i have question regarding use of pthread condition variables. general use case this

//thread 1: pthread_mutex_lock(&mutex); pthread_cond_wait(&cond, &mutex); do_something() pthread_mutex_unlock(&mutex);  //thread 2: pthread_cond_signal(&cond);   

now know pthead_cond_wait unlock mutex , go sleep , when awaken lock mutex , return call how ensure signal thread 2 reaches thread 1 after thread 1 in condition wait. occur thread2 runs first , thread1 causing lost wake up.

if use mutex lock in thread 2 again may happen thread2 lock signal , thread1 still trying acquire lock. again result in lost wake up.

how ensure signal condition variable reaches thread waiting on ?

thanks

you "general use case" not correct. condition variable must paired condition on shared state, known predicate. general use case is:

thread 1:

pthread_mutex_lock(&mutex); while (!condition)     pthread_cond_wait(&cond, &mutex); do_something(); /* requiring 'condition' true */ pthread_mutex_unlock(&mutex); 

thread 2:

pthread_mutex_lock(&mutex); do_something_else(); /* sets 'condition' true */ pthread_cond_broadcast(&cond); pthread_mutex_unlock(&mutex); 

...where condition on shared state protected mutex - example, condition might checking shared queue non-empty.

there can no lost wakeup in structure, because waiting thread always checks condition before waiting. if condition true, doesn't wait - , because holds mutex protecting condition, can't become true between check , wait either.

(note pthread_cond_broadcast() correct function use, , pthread_cond_signal() optimisation applicable in many cases).